Generally, an Orthogonal Frequency Division Multiple Access (OFDMA) system utilizes an Orthogonal Frequency Division Multiplexing (OFDM) technology to realize uplink multiple access.
In an uplink time-frequency domain signal under an Ultra Mobile Broadband (UMB) standard protocol of the 3rd Generation Partnership Project2 (3GPP2), multi-user signals are multiplexed in a manner of a hybrid of OFDMA and Code Division Multiple Access (CDMA) (that is, multiplexed in a manner of OFDMA+CDMA). A device of the signals is superframe, each superframe has 25 physical frames, each physical frame includes 8 OFDM symbols F0 to F7, and F7 is an all 0 signal. In a case that a terminal during initial access is converted from an idle state to an active state or out-of-synchronization, due to mobile characteristics of a user terminal, the user terminal cannot know synchronization information of the system, so that the user terminal needs to acquire initial synchronization information from an Access Network (AN) by sending a random access signal, and initially synchronize with the system according to the initial synchronization information. At the time of initially synchronizing with the system, the terminal further carries partial resource request information, and the AN allocates resources according to the information. The random access signal generally consists of a prefix and a message. Main functions of the prefix are realizing synchronization of uplink signals, and carrying random access information of the user terminal and so on; and the message generally carries connection request information and the like. A random access prefix is also referred to as a random access probe. A UMB protocol specially designs a Reverse Link Access Channel (RACH) for sending the random access probe. When sending an RACH signal, the terminal selects different Walsh sequences according to upper layer information. The length of a Walsh sequence is 1024. Discrete Fourier Transform (DFT) is performed on the Walsh sequence, after being scrambled and interleaved, with a device of 128 sequence elements to acquire 8 corresponding sequences with the length of 128. These sequences are mapped to 128 sub-carriers occupied by RACH channels on F0 to F7, and then sent to a base station through a radio frequency device after being OFDM modulated.
For detection of non-synchronous signals such as RACH signals, signal receiving methods generally adopted are: a frequency domain receiving method and a time domain receiving method.
In an OFDMA system based on the UMB protocol, for a cell with an access range smaller than or equal to 976 m, a universal method for receiving and processing a signal is the frequency domain receiving method. A basic idea of the method is as follows. A receiving end removes the window and the Cyclic Prefix (CP) in a received signal according to synchronization information of a sector in the OFDMA system. DFT is performed on the received signal with the window and the CP removed to transform the signal to a frequency domain at a device of an OFDM symbol time, so as to obtain the received signal on each sub-carrier, extract the received signal of a sub-carrier occupied by an RACH channel, and perform zero interpolation of a certain over-sampling multiplier on the signal to achieve an over-sampling purpose. Inverse Discrete Fourier Transform (IDFT) is then performed to obtain a time domain received signal of the RACH channel, and a cyclic shift is made at a device of an OFDM symbol, where an initial position of the cyclic shift is a delay time of the signal, and the number of times of the cyclic shift is in direct proportion to a maximum time delay (that is, the maximum time delay=the number of the cyclic shift×sampling interval). Afterwards, data after cyclic shift are spliced, the sequence number of a sent sequence is obtained through a conventional detecting and decision method, and thus delay characteristics of a received sequence and information carried by the sequence are obtained.
A basic idea of the time domain receiving method of the prior art is as follows. A time domain filter is adopted to filter a received time domain signal to eliminate interference of the band other than the band of the RACH channel, and then a conventional time domain detecting method is employed to detect the RACH channel to acquire RACH signal time-delay information and access information.
In the implementation of the present invention, the inventors found that the existing non-synchronous signal frequency domain receiving method has at least the following disadvantages, such as great difficulty in detecting the RACH signal with a transmission delay larger than the CP, and high complexity of a receiving end. Moreover, since the existing time domain receiving method needs to design and use a filter, at least the following disadvantage exist: high complexity of the receiving end.